Like bipolar transistors, field effect transistors are widely used as important switches or amplifying elements. A field effect transistor has a structure wherein a semiconductor material is provided with a source electrode and a drain electrode, and via an insulation layer, a gate electrode. The operating characteristics of a field effect transistor are determined by the carrier mobility μ of the semiconductor to be used, the electrical conductivity σ, the electrical capacitance Ci of the insulation layer and the construction of the element (distance L and width W between the source electrode and the drain electrode, the thickness d of the insulation layer, etc.). With respect to the characteristics of the semiconductor material among them, one having a high mobility (μ) shows good characteristics.
At present, silicon is widely used as such a semiconductor material. An inorganic semiconductor represented by silicon is required to be treated at a high temperature of at least 300° C. during the production, whereby it is difficult to employ a plastic substrate or film as the substrate, and it has a drawback such that a large energy is required for the production. Further, it requires an element preparation process in vacuum, whereby an expensive installation is required for the production line, thus leading to a drawback of high costs.
Whereas, a transistor employing an organic semiconductor can be produced by a lower temperature process than the inorganic semiconductor in most cases, whereby it is possible to employ a plastic substrate or film as the substrate, and it is possible to prepare an element which is light in weight and scarcely breakable. Further, there may be some whereby an element can be prepared by employing a printing method or coating of a solution, and it is possible to produce an element having a large area at a low cost. Further, the material is rich in variation, and it is possible to easily basically change the characteristics of the material by changing the molecular structure, and accordingly, by combining different functions, it is also possible to realize an element having a function which can not be attainable by an inorganic semiconductor.
With respect to a transistor employing an organic semiconductor as the semiconductor, JP-A-61-202469 discloses one employing a conductive polymer or a conjugated polymer, and Japanese Patent 2,984,370 discloses one employing a low molecular weight compound.
Typical structures of conventional transistors employing an organic semiconductor as the semiconductor, are shown in FIGS. 1 to 3.
In the field effect transistor in FIG. 1, a gate electrode 2 is provided on a support substrate 1, and further, an insulation layer 3 and an organic semiconductor layer 4 are provided thereon. So as to contact the organic semiconductor layer 4, a source electrode 5 and a drain electrode 6 are provided on the insulation layer 3. This field effect transistor is referred to as a bottom gate/bottom contact type.
The field effect transistor in FIG. 2 is different from the field effect transistor shown in FIG. 1 in that a source electrode 5 and a drain electrode 6 are provided on an organic semiconductor layer 4 on an insulation layer 3, and it has the same construction except for this difference. This field effect transistor is referred to as a bottom gate/top contact type.
In the field effect transistor shown in FIG. 3, a source electrode 5 and a drain electrode 6 are provided on a support substrate 1, and an organic semiconductor layer 4 and an insulation layer 3 are laminated on the support substrate 1, and a gate electrode 2 is provided on the insulation layer 3. This field effect transistor is referred to as a top gate/bottom contact type.
With such field effect transistors, when a voltage is applied to the gate electrode 2, the carrier density in the organic semiconductor layer 4 will be changed in the vicinity of the interface between the organic semiconductor layer 4 and the insulation layer 3, thereby to change the electric current flowing between the source and drain electrodes 5 and 6.
In such a field effect transistor employing an organic semiconductor (hereinafter sometimes referred to as “an organic filed effect transistor”), as mentioned above, when a plastic substrate or film is employed as the support substrate, it is possible to realize a transistor which is flexible and scarcely breakable. Such an organic field effect transistor may be used as a switching element in a flexible display, as disclosed in Bell Lab. Lucent Technologies, PNAS., 98,4835.
However, even in a case where a plastic substrate or film is employed as the support substrate, if, for example, there is a substantial difference in the mechanical characteristics between the support substrate and e.g. the insulation layer, it is likely that when a stress is exerted to the organic field effect transistor, the support substrate may be deformed and then return to the initial shape when the stress is lifted, but the insulation layer may not return to the initial shape, whereby the function as the element may be destroyed. However, no detailed study has been carried out with respect to the mechanical characteristics as such a flexible element, and the characteristics of the element.